rom/mathffp/spfloor.c

   1 /*
   2     Copyright © 1995-2004, The AROS Development Team. All rights reserved.
   3     $Id$
   4 */
   5
   6 #include "mathffp_intern.h"
   7
   8 /*
   9     FUNCTION
  10         Calculate the largest integer ffp-number less than or equal to
  11         fnum
  12
  13     RESULT
  14         FFP number
  15
  16         Flags:
  17           zero     : result is zero
  18           negative : result is negative
  19           overflow : 0 (???)
  20
  21     NOTES
  22
  23     EXAMPLE
  24        floor(10.5) = 10
  25        floor(0.5)  = 0
  26        floor(-0.5) = -1
  27        floor(-10.5)= -11
  28
  29     BUGS
  30
  31     SEE ALSO
  32         @Math.Floor@
  33
  34     INTERNALS
  35       ALGORITHM:
  36         <p>The integer part of a ffp number are the left "exponent"-bits
  37         of the mantisse!
  38         Therefore:
  39         Test the exponent for <code><= 0</code>. This has to be done separately!
  40         If the sign is negative then return -1 otherwise return 0.</p>
  41
  42         <p>Generate a mask of exponent(y) (where y is the given ffp-number)
  43         bits starting with bit 31.
  44         If <code>y < 0</code> then test whether it is already an integer. If not
  45         then y = y - 1 and generate that mask again. Use the
  46         mask on the mantisse.</p>
  47
  48     HISTORY
  49 */
  50
  51 AROS_LH1(float, SPFloor,
  52     AROS_LHA(float, y, D0),
  53     struct LibHeader *, MathBase, 15, Mathffp
  54 )
  55 {
  56     AROS_LIBFUNC_INIT
  57
  58     LONG Mask = 0x80000000;
  59
  60     if (((char)y & FFPExponent_Mask)  <= 0x40)
  61     {
  62         if ((char)y < 0)
  63         {
  64             SetSR(Negative_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  65             return 0x800000C1; /* -1 */
  66         }
  67         else
  68         {
  69             SetSR(Zero_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  70             return 0;
  71         }
  72     }
  73
  74     /* |fnum| >= 1 */
  75     Mask >>= ( ((char) y & FFPExponent_Mask) - 0x41);
  76     Mask |= FFPSign_Mask | FFPExponent_Mask;
  77
  78     /* fnum is negative */
  79     if ((char) y < 0)
  80     {
  81         /* is there anything behind the dot? */
  82         if (0 != (y & (~Mask)) )
  83         {
  84             Mask = 0x80000000;
  85             y    = SPAdd(y, 0x800000c1); /* y = y -1; */
  86             Mask >>= ((char) y & FFPExponent_Mask) - 0x41;
  87             Mask |= FFPSign_Mask | FFPExponent_Mask;
  88         }
  89     }
  90
  91     if((char) y < 0)
  92     {
  93         SetSR(Negative_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  94     }
  95
  96     return y & Mask;
  97
  98     AROS_LIBFUNC_EXIT
  99 }